Look good is variable that depends on viewing distance. Viewing distance is variable and depends on print size. It's a very strange way that leads to non-exact figures for "looks good".

300ppi resolution for instance is a standard print resolution specified mostly for 6x4 and 8x5 prints which are typically viewed at arms lengthish. If you print a 40" wide image then you won't need to print at 300ppi because to actually look at the image properly will require you to step back. Double the distance back and you half the resolution you need.

On top of this all of these numbers assume perfect vision. Nanna will assume your 100ppi images are absolutely gorgeous

I have made prints at 16x20 at 140ppi that are perfectly sharp, even up close. You might be able to get away 100ppi on that size image as long as you don't resample to get to that resolution. At 100ppi your 1000x1000 pixel image could print at 10x10 inches.

... say a photo is 1000 pixels by 1000 pixels, 300 resolution - how do I find out the maximum print size that it will still look good and not go bad? ...

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You haven't provided enough information to calculate. There is absolutely no possible way to answer without also specifying the viewing distance.

The common wisdom (reasonably accurate but not precise) is that for a print made on a premium printer and to be viewed at normal reading distance needs to be in the range of 300 pixels per inch (that's actual ppi on the print and NOT necessarily the ppi setting spec'd in the digital file).

In fact, very few people will detect a difference between 240ppi and 300ppi and almost no one will notice the difference between 150ppi and 300ppi unless they are comparing prints side by side, especially when the print is large. When the viewing distance is longer, lower original resolutions are fine though when an image drops below 150ppi it is generally best to upsample it at least 150 before printing if you have good image processing software.

For practical application, take Mpix for example. Their printers output at 250ppi and the recommended minimum is 100ppi. The resolution setting on your file will not effect the print size, it only changes how the image is displayed on your screen. For printing only the total number of pixels and viewing distance will determine how large the print can be.

Sigh. Americans. You know I actually ended up leaving a bag at home one day when an American friend asked me where he should put it and I said the "boot of the car". He didn't know what the boot was so just left the bag in my room. "Oh you meant the trunk" is no consolation for we need need to go back and get it! :lmao:

Sigh. Americans. You know I actually ended up leaving a bag at home one day when an American friend asked me where he should put it and I said the "boot of the car". He didn't know what the boot was so just left the bag in my room. "Oh you meant the trunk" is no consolation for we need need to go back and get it! :lmao: